1. Field of the Invention
The present invention relates to a method and a device for determining a temperature progression of a link capacitor of a link converter that contains at least one n-phase inverter. Furthermore, the use of the methods according to the invention to determine the remaining service life of the link capacitor from the temperature progression is described.
2. Discussion of Background Information
Link converters are used in a large number of circuits, including servo amplifiers, frequency converters, converters, feeders/feedback devices, etc., particularly for electric motor drives, for example stepper motors or brushless direct-current motors, etc. Capacitors, particularly electrolytic capacitors (e-caps), are often installed in links as energy-storing elements, wherein these link capacitors are used to smooth the link voltage. In some systems, only a limited number of such capacitors can be installed because of desired space saving. The service life of the whole circuit or of circuit parts is often dependent primarily on the service life of the capacitor of the link. Because thermal problems can occur in the link capacitors in the case of high power requirements, it is advantageous if the remaining service life of the link capacitor is known. In turn, the capacitor service life can be determined from a temporal temperature progression of the capacitor in a known manner by means of the Arrhenius formula. The ambient temperature and/or the ripple current load of the capacitor is often used for this purpose. Therefore, it is usually possible to determine the capacitor service life only in the laboratory, because only there can defined conditions (ambient temperature, current progression, etc.) be created. This approach is often used for the a priori design and optimization of capacitors. In the case of known requirements, capacitors having desired properties can then be used.
However, especially in the case of requirements not known beforehand, it is often desirable that the (remaining) service life of a capacitor can be predicted or determined also during ongoing operation, and in particular while the capacitor is installed in a link converter. KR 2013 0110553 A provides such an approach, wherein in the document the converter supplies an electric motor. The total electrical power of the converter circuit is calculated on the basis of the torque of the electric motor and the angular velocity of the associated rotor. Subsequently, the ripple current is estimated on the basis of the voltage present at the link capacitor and the determined total power. Then the progression of the temperature of the link capacitor is determined from the progression of the estimated ripple current on the basis of stored characteristic curves, and subsequently the service life of the link capacitor is determined. A disadvantage in this case is that the determined total power of the converter does not correspond to the power consumed at the link capacitor and therefore subsequently the estimated ripple current does not actually correspond to the current through the link capacitor. Therefore, the determined temperature progression also is highly inaccurate. Furthermore, additional sensors are required to determine the torque and the velocity.
The capacitor current and the capacitor voltage or the power consumed at the link capacitor or the capacitor temperature could be determined by measurement. However, the entire additional measurement equipment required for this purpose is often undesired in a link converter for reasons of cost of space. In particular, the measurement of the capacitor temperature (i.e., the temperature inside the link capacitor) would require special link capacitors that are equipped with integrated temperature sensors and are therefore expensive.